# Regular Expression - Group (Capture|Substitution)

> Procedural Languages > Multilingual Regular Expression Syntax (Pattern)

### Table of Contents

## 1 - About

`group`

are regexp expression that normally capture the match during the parsing. You can then (extract|reference) the match content.

Groups are inside parentheses.

Look-around are also `groups`

but implements an assertion and are not capturing the content

## 2 - Articles Related

## 3 - Syntax

Every group must begin with an open bracket and end with a close bracket.

(myRegexp0 ( myRegexp1) ( myRegexp2) )

Construct | Definition |
---|---|

`(?<name>X)` | X, as a named-capturing group |

Non-Capturing | |

`(?:X)` | X, as a non-capturing group |

`(?>X)` | X, as an independent, non-capturing group |

Assertion (See Regexp - Look-around group (Assertion) - ( Lookahead | Lookbehind )) | |

`(?=X)` | X, positive lookahead (via zero-width) |

`(?!X)` | X, negative lookahead (via zero-width) |

`(?<=X)` | X, positive lookbehind (via zero-width) |

`(?<!X)` | X, negative lookbehind (via zero-width) |

Flag | |

`(?idmsuxU-idmsuxU)` | Nothing, but turns match flags i d m s u x U on - off |

`(?idmsux-idmsux:X)` | X, as a non-capturing group with the given flags i d m s u x on - off |

## 4 - Index

Capturing groups are numbered by counting their opening parentheses from left to right.

In the expression `((A)(B(C)))`

, for example, there are the following groups:

- 0 - Group zero always stands for the entire expression -
`((A)(B(C)))`

- 1 -
`((A)(B(C)))`

- 2 -
`(A)`

- 3 -
`(B(C))`

- 4 -
`(C)`

## 5 - Non-Capturing

### 5.1 - Basic

A non capturing group will not be indexed.

In the expression `(?:A)(B)(C)`

, for example, there are the following groups:

- 0 - Group zero always stands for the entire expression -
`(?:A)(B)(C)`

- 1 -
`(B)`

- 2 -
`(C)`

The group `(?:A)`

was not captured.

### 5.2 - Look-around

## 6 - Substitution

When you want to use the content of each captured group, you will generally use the following substitution construct:

`${n}`

for the group index`${groupName}`

for the group name

When using group index, this construct must be used when:

- the number of group is greater than 9
- you want a number that follow the substitution

The dollar is also not always mandatory:

`$n`

for the group index`$groupName`

for the group name

Their is also a shorthand notation for groups up to 9.

Symbol | Definition |
---|---|

`\0` | backreference to the entire expression |

`\1` | backreference to group 1 |

`\2` | backreference to group 2 |

`\n` | backreference to group n |

## 7 - Example

The below regular expression has two groups

([^ ]) (.*)

where:

- the first group is
`[^ ]`

which will parse all non space characters. - the second group is
`.*`

which will take all characters.

if you parse the following text:

Hello World

You will get:

- in the first group,
`\1`

, the text`Hello`

- and in the second group,
`\2`

, the text`World`

See more example here: Notepad++ - Replace with Regular Expression